Vibration damped sandwich systems



Dec. 15, 1970 H. OBERST Er VIBRATION DAMPED SANDWICH SYSTEMS Filed Nov.18, 1968 2 Sheets-Sheet FIG. 20

Lllllllllllllllllllll FIG. 2b

INVEN TOPS HERMANN OBEF'ST JOACHIM EBIGT GU N THEE" [JUV El BY I 744M '0Ar "Jr NE Y8 United States Patent ()fifice 3,547,755 Patented Dec. 15,1970 Int. (:1. B32b /05; C08f 19/02; E04b 1/99 Us. or. 161-165 4 ClaimsABSTRACT OF THE DISCLOSURE A vibration damped sandwich system comprisingtwo hard plates and interposed between the plates a vibration dampinginterlayer comprising a graft polymer of 66% by weight of styrene and34% by weight of acryonitrile on copolymers of about 30 to 50% by weightof vinyl acetate and about 70 to 50% by weight of ethylene.

The present invention relates to vibration clamped sandwich systemshaving interlayers made of a graft polymer of a mixture of styrene andacrylonitrile on copolymers of vinyl acetate and ethylene.

It is known from South African specification No. 5,269 that highlyvaluable vibration damping materials of a broad temperature bandsuitable for damping bending vibrations of metal sheet constructions canbe prepared by copolymerizing monomers whose homopolymers difier intheir second order transition temperature by at least 20 C. The abovespecification also reports that as vibration damping materials having abroad temperature band there can be used, among others, predominantlyamorphous copolymers of ethylene and vinyl esters of fatty acids with 2or 3 carbon atoms, for example vinyl acegate/ethylene copolymers. Theethylene/vinyl acetate copolymers used up to now are, however, notsatisfactory in all respects.

, It has now been found that graft polymers of a mixture of 66% byweight of styrene and 34% by weight of acrylonitrile on copolymers ofvinyl acetate and ethylene have outstanding damping properties and avery broad temperature range of damping and are, therefore, especiallysuitable for the vibration damping of sandwich systems of hard plates,in particular metal sheets. Suitable copolymers of vinyl acetate andethylene are preferably those containing about to 50% by weight of vinylacetate, especially about 40% by weight of vinyl acetate and,consequently, about 70 to 50%, especially about 60% by weight ofethylene. From the economical point of view they have the advantage ofbeing rather cheap.

The graft polymers of a mixture of 66% by weight of styrene and 34% byweight of acrylonitrile on copolymers of vinyl acetate and ethylene areobtained by preparing a gel of the specified copolymer in a mixture of66% by weight of styrene and 34% by weight of acrylonitrile containing acatalyst and performing a free-radical initiated polymerization at atemperature in the range of from about 60 to about 180 C. Especiallygood results are obtained with graft polymers of 55 to 90% by weight ofa mixture of 66% by weight of styrene and 34% by weight of acrylonitrileon 45 to 10% by weight of a copolymer as described above, for example agraft polymer of 60% by weight of a mixture of 66% by weight of styreneand 34% by weight of acrylonitrile on 40% by weight of a copolymer asspecified above. As polymerization initiator there may be used tertiarybutylhydroperoxide in the usual concentrations. With graft polymers ofthis type, the vibration damping effect of which critically depends onthe weight proportion of the monomers, very broad damping curves can beobtained having high maximum damping values.

The present invention thus provides sandwich systems of hard plates, inparticular metal sheets, having vibration damping, self-adherentinterlayers consisting of graft polymers of a mixture of 66% by weightof styrene and 34% by weight of acrylonitrile on copolymers of vinylacetate and ethylene, for which interlayers there are used graftpolymers of 55 to by Weight of a mixture of 66% by weight of styrene and34% by weight of acrylonitrile on 45 to 10% by weight of a copolymer of30 to 50% by weight of vinyl acetate and 70 to 50% by weight ofethylene.

FIGS. la and 1b of the accompanying drawings are plots againsttemperature of the loss factor d to illustrate the superior eificiencyof the novel systems. The curve in FIG. 1a shows the loss factor dcombof a metal sheet arrangement of the invention as a function oftemperature. For comparison, one of the most effective vibration dampingmaterials known for metal sheet arrangements was used, namely acopolymer of 63% by weight of vinyl acetate and 37% by weight of dibutylmaleate containing as plasticizer 15% by weight of 2-ethylhexylphthalate and 15% by weight of tricresyl phosphate, calculated on themixture (curve FIG. lb). The copolymer of curve 111 was a thermoplasticadhesive especially suitable for producing vibration damped metal sheetsandwich systems comprising two outer metal sheets and a self-adherentthermoplast as damping interlayer. Systems of this type provide adamping effect which is extremely high in its maximum and cannot beexceeded for physical reasons (cf. H. Oberst and A. Schommer,Kunststofie, vol. 55, p. 634 (1965), especially FIG. 9). In asymmetrical arrangement comprising two metal sheets, each having athickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, theloss factor dcomb of the combined system, measured in the bending wavemethod (of. for example H. Oberst, L. Bohn and F. Linhardt, Kunststoife,'vol. 51, page 495 (1961)), almost reaches the value dcomb of 1. Theknown metal sheet damping by one-side damping coatings which are appliedby spraying, trowelling or bonding in the form of layers of so-calledvibration damping materials show loss factors generally of less than d=0.2 with technically reasonable thicknesses or ratios of coating massto plate mass of the combined system. With metal sheet sandwich systemswhich gain growing importance in recent times, it is possible to obtaindamping values that are increased by a multiple, as shown by the presentexample, when the interlayer material has the appropriate compositionand thickness.

The temperature band width of the damping of the metal sheet sandwichsystem does not only depend on the viscoelastic properties of theinterlayer and the steel sheets but also to a considerable extent on thegeometry of the arrangement, i.e. on the ratio of the layer thicknesses(cf. loc cit. (1965)), (FIGS. 8 to 10). With metal sheet sandwichsystems the band width is advantageously defined as the range of thetemperature interval within which the value d =0.05 is exceeded. Thedamping of metal sheets which are not damped by additional vibrationdamping means in metal sheet constructions of various types correspondsto values d 0.01. The reference value dcomb of 0.05 thus means aconsiderable increase in the damping eifect (by about 15 db (decibel))as compared to the nil damping d,,,, =0.0l.

In the curve shown in FIG. 1b the reference value dcomb of 0.05 isexceeded in the mainly interesting frequency range of from to 1,000c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. Thetemperature band width thus corresponds to about 50 C. Sandwich systemsof this type are suitable for many technical fields of application. Bymodifying the content of plasticizer, it is possible to shift thetemperature band of a high damping effect to higher temperatures andthus to adapt the material to sepcial technical uses, for example inmachine units operating at elevated temperatures. This example of ametal sheet sandwich system comprising a self-adherent interlayer havingoptimum properties of a vibration damping material with a broadtemperature band prepared by copolymerizing appropriate monomericcompounds has hitherto not been surpassed by other arrangements ofsimilar kind and may be taken as standard for judging the acousticefliciency of the system according to the invention.

FIGS. 1a and lb show the temperature curves of the loss factor dcomb ofmetal sheet sandwich systems comprising steel sheets of a thickness of0.5 millimeter each and damping interlayers of a thickness of 0.75 and0.3 millimeter, respectively, (FIG. 1b) for a frequency of 100 c.p.s.and 1,000 c.p.s.

The curves were measured with sandwich systems the interlayers of Whichconsisted of:

The arrangement 1a, whose monomer proportion lies in the optimum rangehas a very broad temperature band width with relatively high maximumdamping values. With the arrangement 1a the center of damping is atabout 80 C. to 120 C. for 100 c.p.s. and at about 5 C. for 1,000 c.p.s.with maximum damping values of about 0.3. The temperature band width isabout 215 C. for 100 c.p.s. and about 135 C. for 1,000 c.p.s. Inarrangement 1a, the slow decrease of the damping towards hightemperatures and with low frequencies of 100 c.p.s. also towards lowtemperatures is especially favorable. The excellent vibration dampingproperties are maintained, with reference to d =0.05, within atemperature range of about C. to about 165 C. As compared with thestandard system 1b, arrangement 1a has an appreciably broadertemperature band width and better vibration damping properties attemperatures of about 0 C. and above all at a temperature above 45 C.,so that arrangements of this type are suitable for quite a number ofapplications at normal and elevated temperature (for example machineunits and appliances operated at elevated temperature).

A special advantage of the vibration damping material of the presentinvention resides in the fact that it may be applied continuously duringthe mass production of the metal sheet sandwich systems.

For this purpose it can be applied (1) in the form of the finished graftpolymer and (2) as a gel of the copolymer in the styrene/acrylonitrilemixture to be grafted on, which contains the polymerization initiator.In the latter case the graft polymer is produced by subjecting thesandwich system comprising the gel as interlayer to a thermal treatmentat a temperature in the range of from about 60 to about 180 C. It is athermoplastic adhesive which may be applied to the metal sheets bytrowelling, brushing or pouring at elevated temperature. The sandwichsystem may then be advantageously cooled under pressure between rollers.Except for degreasing the metal sheets do not require a preliminarytreatment ther adhesive. The adhesionisvery good.

The vibration damping material of the present invention has a goodresistance to flow. The metal sheet sandwich system may, within broadlimits, be processed as normal metal sheets, that is they may becreased, bent, shaped, welded and riveted. In this manner laminatedsystems are obtained having a damping height and temperature range ofdamping which makes them. well suitable for many applications at normaland elevated temperatures.

Minor amounts of fillers, for example for improving the electricconductivity (improvement of resistance welding) may be incorporated inthe vibration damping materials. In order not to affect the dampingeffect adversely it is advantageous to use less than 1% by weight,preferably less than 0.5% by weight of filler, calculated on thepolymer. Suitable fillers are, for example, heavy spar, silic'ic acid,graphite and soot.

The metal sheet sandwich system suitably has a total thickness in therange of from 1'to 6 millimeters. The interlayers may have a thicknessof 0.1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximumdamping effect is obtained with symmetrical laminated systems. With anequal weight, asymmetrical laminated systems have, however, a higherstiifness in flexure and strength. Asymmetrical laminated systems are,therefore, preferred for those applications which require a highstrength with respect to the weight. The ratio of the thicknesses of theouter plates or metal sheets is preferably within the range of from 1:1to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems withsymmetrical arrangement (a) and asymmetrical arrangement (b) in whichthe interlayer 2 is interposed between the two outer plates or metalsheets 1.

What is claimed is: Y I

1. A vibration damped sandwich system comprising two hard plates andinterposed between the plates a vibration damping interlayer comprisinga graft polymer of 55 to 90% by Weight of a mixture of 66% by weight ofstyrene and 34% by weight of acrylonitrile on 45 to 10% by weight of acopolymer of 30 to 50% by weight of vinyl acetate and 70 to 50% byweight of ethylene.

2. A vibration damped sandwich system as claimed in claim 1, wherein thehard plates are metal sheets.

3. A vibration damped sandwich system as claimed in claim 1, wherein thegraft polymer of the interlayer contains up to 1% by weight of a filler,calculated on the graft polymer.

4. A vibration damped sandwich system as claimed in claim 1 wherein theratio of the thicknesses of the plates lies in the range of from 1:1 to1:4.

and fur- References Cited UNITED STATES PATENTS 3,218,373 11/1965Salyer. 3,271,188 9/1966 Albert et al. 3,314,904 4/ 1967 Burkus.3,355,516 11/1967 Hardt et al. 3,399,103 8/1968 Salyer et al. 3,414,63812/1968 Hardt et al. 3,433,724 3 /1969 Chapiro et al.

JOHN T. GOOLKASIAN, Primary Examiner G. W. MOXON II, Assistant ExaminerUS. Cl. X.R. 161-218;181--33;260--80.81,873,878

